An r/gameai developer building an NPC with robotics-style perception went looking for prior art and found that no shipped game keeps a probability distribution over where the player is. The thread — part technical question, part literature review — argues that shipped game AI represents belief discretely, and the probabilistic machinery stayed in robotics and search theory.

The OP’s approach:

  • His NPC maintains a full occupancy grid over player position; noisy perception events update it, and a diffusion step spreads probability mass along walkable cells between observations
  • It is a standard Bayes filter, the kind of thing robotics uses for target tracking
  • The gap is load-bearing for him: “if it is wrong my project has no gap to sit in”

The prior-art case for discrete belief:

  • Orkin’s F.E.A.R. paper is about goal-oriented action planning, not belief
  • Isla’s Halo 2 paper is the closest published model: a discrete remembered target position and orientation plus awareness levels, with actors allowed to believe things that are not true
  • Both are fallible belief; neither is a distribution over space

The replies mostly confirm the thesis, with one caveat:

  • Alien: Isolation’s director AI is the closest shipped analogue — a meta-AI that tracks the player but constrains itself so the alien behaves believably
  • In-engine NPCs have no sensor noise by default; the engine knows the exact position, and games fake perception limits with vision cones and low-energy sensor malfunction instead
  • The sharpest pushback is economic, not technical: solving for noise is “computationally suicidal” in a clean room — games already have ground truth, so simulate whatever perception failure creates the feeling you want and skip the Bayesian machinery

The takeaway is a clean line for AI-feeling characters: realism is not the goal, legible fallibility is. Discrete belief — last-known position plus awareness levels — is cheaper, easier to debug, and lets designers author exactly which lies an NPC should believe. Probability fields earn their keep only when genuine sensor noise is part of the fantasy.